This invention relates generally to light-emitting devices, and more particularly, to superluminescent light-emitting diodes. Light-emitting diodes are well known semiconductor devices in which an electrical current is passed through a diode junction and produces light emission in an active layer of semiconductor material at the junction. At least one facet of the device is coated with an anti-reflective material, through which light is emitted. This is to be contrasted with a laser diode, in which stimulated emission of light occurs, and there are usually two opposed reflective facets, and there are repeated reflections of light between the facets before a coherent laser beam emerges. The resulting laser beam usually has a very narrow spectral width, i.e. it is practically monochromatic.
Light-emitting diodes operating at relatively high powers and having a relatively broad spectral width are called superluminescent light-emitting diodes. There is a need for these devices in fiber optic systems having a requirement for low modal noise, such as in fiber optic gyroscopes. Commercially available superluminescent light-emitting diodes emit light at powers as high as 4-6 mW (milliwatts). However, when the power in these devices is increased above about 1-2 mW, the frequency spectrum is substantially narrowed. Driving the devices to higher powers may eventually cause lasing, in spite of the presence of the anti-reflective coating, since even the best anti-reflective coating will reflect some proportion of the light impinging on it, and lasing will eventually occur if the power is increased to a high enough level. For this reason, the anti-reflective coatings in superluminescent light-emitting diodes have to be carefully controlled to permit operation at higher power levels.
As the power of a superluminescent light-emitting diode is increased and its spectral width is consequently decreased, the coherence length of light from the device is increased. The coherence length is another measure of the spectral purity of light. As the spectral width becomes narrower, the coherence length increases. Some applications of superluminescent light-emitting diodes, such as fiber optic gyroscopes, require low modal noise, which means that the light source must have a low coherence length and correspondingly wide spectral width. Prior to this invention, available superluminescent light-emitting diodes were unable to meet this requirement.
It will be appreciated from the foregoing that there is still a need for a superluminescent light-emitting diode that can be operated at higher powers without spectral narrowing. The present invention satisfies this need.